The MRI apparatus is an apparatus that measures an NMR signal, which is generated by the spins of nuclei that form an object when irradiating the object with a high-frequency magnetic field pulse (hereinafter, referred to as an RF pulse), and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding and different frequency encoding are given to NMR signals by the gradient magnetic field, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
In the above-described MRI apparatus, when irradiating a desired part of an object with an RF pulse, an RF transmission coil is disposed around the desired part and an RF pulse having a nuclear magnetic resonance frequency, which is determined according to the static magnetic field strength and the excitation target nuclide, is supplied to the RF transmission coil. Therefore, design or adjustment (tuning) of the RF transmission coil is performed so that the resonance frequency of the RE transmission coil becomes a nuclear magnetic resonance frequency that is the frequency of the RE pulse.
Among the above-described RF transmission coils, a BC coil is used especially in the horizontal magnetic field type MRI apparatus.
In the case of a cylindrical BC coil, a plurality of linear conductor elements are disposed at equal distances along the circumference of the cylindrical shape in parallel with the central axis of the cylindrical shape, and circular ring-shaped conductor elements electrically connected to the plurality of linear conductor elements are disposed at both ends of the BC coil. In this ring-shaped conductor element, a plurality of resonance capacitor elements (capacitors) having the same capacitance are inserted in series, so that the capacitance of each of these resonance capacitor elements is adjusted at the time of adjustment (tuning) of the Q value or the resonance frequency of the RF transmission coil.
PTL 1 discloses a multilayer structure of an outside (external) ring and an inside ring in a two-frequency tuning birdcage-type coil. An example of performing tuning by appropriately setting the inductance of the outside ring or the capacitance between the inside ring and the outside ring in this coil is disclosed.